Method &amp; apparatus for the distribution of network traffic

ABSTRACT

A packet network device, such as a router or switch, includes functionality that operates to receive network traffic, process the traffic as needed and to forward the traffic to its destination. Additionally, each router includes a weighted equal cost multipath routing function that operates to identify equal cost paths over which to forward the network traffic, to calculate a path weighting that is dependent upon the path bandwidth and to forward the traffic ingressing to it over each of the equal cost paths according to the calculated path weighting.

BACKGROUND

1. Field of the Invention

The present disclosure relates generally to packet network devices suchas switches and routers, and more particularly to methods for theoptimal and dynamic, global distribution of traffic ingressing to anetwork system over multiple paths.

2. Description of Related Art

A network system operating according to the Internet Protocol (IP) istypically comprised of some number of network systems (NS), such as theNS 100 shown in FIG. 1. The term network system and autonomous systemare interchangeable in this context. Up until recently, an AS wasconsidered to be a set of routers under the administration of a singleentity, using an interior gateway protocol and using common metrics toroute packets within the AS. More recently, it has become common for asingle AS to employ two or more interior gateway protocols (IGP) andseveral sets of metrics. From one perspective, an AS can be consideredto be a connected group of one or more IP prefixes, run by one or morenetwork operators, which has a single, clearly defined routing policy.

The NS 100 of FIG. 1 includes a number of edge routers (ER1-ERn)connected to a core network. The core network is comprised of aplurality of core routers (CR), CR₁ to CR_(n), that operate to forwardtraffic received from one of the edge routers (ER1-ERn) to another corerouter or to another one of the edge routers (ER1-ERn). All of the ERsare connected to at least one core router by one or more physical orlogical links. Each of the ERs is capable of receiving traffic fromoutside the NS 100 and sending this traffic to the core network where itis forwarded to an ER for transmission outside the NS. Based on thetopology of NS 100, multiple paths through the NS can be calculated fortraffic ingressing on any of the ERs.

In FIG. 1, a flow of traffic labeled T_(i/o), ingresses to or egressfrom ER₁, and this traffic T_(i/o), can be distributed by the routerscomprising NS 100 in proportions D1, D2 and Dn to each of a plurality ofthe ERs, ER2, ER3 and ERn respectively. Each portion D1, D2 and Dnrepresents a certain amount of traffic that is typically measured inbits of information per second, for instance, and each portion can bethe same or different amounts of traffic. As shown in FIG. 1, theportion D1 can be distributed over a path P1, portion D2 can bedistributed over a path P2 and portion Dn can be distributed over a pathPn through the NS 100. Each of the paths, P1-Pn, can be comprised of asequence of multiple routers connected by the physical or logical links,and each of the links are capable of supporting a particular amount oftraffic. While the links connecting the routers in NS 100 are shown assingle links, each of the links can be either single physical links oran aggregation of two or more logical links. Each of the links cansupport a particular volume or amount of network traffic, which isreferred to as link bandwidth. The capability of a network link tosupport a particular volume of network traffic is determined by thecapacity of physical interfaces connected to a link to process thevolume of traffic. Physical interfaces included on a router can bedesigned to process traffic ingressing to them at various rates, whichcurrently can approach 40 Gbits/second. The amount of traffic that alink can support is typically referred to the link bandwidth, and theunused or available link bandwidth at any point in time is referred toas instantaneous available link bandwidth or simply available linkbandwidth. Path bandwidth is the minimum of the link bandwidths oravailable link bandwidths of all of the links comprising a path throughthe network system. So for example, network traffic T_(i/o) can beforwarded along the path P1 which includes ER1 (ingress router), corerouter CR0 and ER2 (egress router), and the available bandwidth overpath P2 is the minimum link bandwidth along the path P1. In this case,path P1 includes a link, L1, that connects ER1 to CR0 and a link, L2,that connects CR0 to ER2. If the bandwidth of link L1 is 10 Gbits/secondand the bandwidth of link L2 is 5 Gbits/second, then the path P1bandwidth is lesser of the two link bandwidths, or 5 Gbits/second.

In order to forward the traffic T_(i/o), over path P1 in the NS 100without the loss of any information, it is necessary for the availablebandwidth of path P1 to be greater than or equal to the volume or amountof traffic in T_(i/o). Assuming that the available path P1 bandwidth isequal to or greater than the volume of traffic in T_(i/o), if the NS 100is stable along path P1 (i.e., the link states comprising the path arenot changing), the traffic T_(i/o), can be forwarded over path P1without the loss of any information. However, in the event that one ormore internal ports associated with a link comprising path P1 flaps(fails), the available path P1 bandwidth may be lowered, resulting inthe loss of some of the traffic T_(i/o) until the routers comprising NS100 can recalculate a new path and program their forwarding tables toredirect some or all of the traffic T_(i/o). Prior art trafficredistribution methods are limited in as much as the network protocolrunning on each router in the system only considers the traffic T_(i/o)ingressing to it when recalculating a route through the network system.

Interior Gateway Protocols (IGP) running on routers or switches in anetwork system operating according to the Internet Protocol (IP)generally operate to collect certain information from neighboringrouters and switches that can be used to calculate paths through thenetwork that are used to forward network traffic. As described earlierwith reference to FIG. 1, a path can be comprised of a sequence ofmultiple routers connected by physical or logical links, and each of thelinks are capable of supporting a particular amount of traffic.Depending upon the complexity of the network system, there can bemultiple paths between two different network edge devices, such as theERs of FIG. 1. Typically, an IGP, such as the well known OSPF (OpenShortest Path First) protocol, uses a cost metric associated with eachrouter interface (physical or logical) to calculate one or more shortestpaths from the router to a destination. The cost metric can be assignedto each interface by a system administrator and this cost metric candependent on the distance from one router to another (round-trip time),link bandwidth, link availability (delay), and/or link reliabilityfactors to name only three criteria that can be considered whenassigning cost to a router interface. The OSPF protocol running on arouter uses the costs assigned to each of its interfaces to calculatethe shortest paths from it to a destination address, for instance.Specifically, the Dijkstra algorithm is typically used to calculate theleast cost paths through a network system, such as the network system100 in FIG. 1. The result of applying the Dijkstra algorithm to linkstate information maintained by each router is a series of connectedrouters that represent the least cost paths to each router and the costof each path.

Referring again to FIG. 1, if the result of the calculation to identifythe least cost paths from ER1 to ER3 in the NS 100 result in a path P2cost equal to three and a path P3 cost equal to three, then OSPF runningon ER1 will typically select either path P2 or path P3 (assuming P2 andP3 have enough available bandwidth to support the traffic) as the pathsfor traffic T_(i/o) through the NS 100. Paths P2 and P3 are in this caseconsidered to be equal cost paths, and the routing technique mostcommonly employed to select which of two or more equal-cost paths toforward a flow of traffic is the well known Equal Cost Multi-Path (ECMP)routing technique. ECMP is a routing technique that is explicitlysupported by the OSPF protocol. A number of different methods can beused to determine which of several equal cost paths or next hops areselected. Hash-threshold is one method for determining which of severalequal cost next hops to select and the round-robin method is another.Each method has their advantages and disadvantages and the reasons forselecting one of the other method is not discussed here. ECMP routingtechniques typically divide the traffic with a common destinationequally among the multiple equal cost paths, regardless of the bandwidththat is available on any one of the equal cost paths and regardless ofthe technique employed to select the traffic transmission path.

Continuing to refer to FIG. 1, assuming that the traffic_(i/o) is beingforwarded over two equal cost paths, paths P2 and P3 for instance, andthat the available bandwidth on path P2 is 1 Gbit/second and that theavailable bandwidth on path P3 is 2 Gbits/second, if ECMP routingdistributes traffic T_(i/o) equally between paths P2 and P3, and if aport associated with the link L5 comprising path P2 flaps (assuming L5is a logical link comprised of multiple physical links), then dependingupon whether path P2 is oversubscribed or not, some traffic may bedropped from that portion of the traffic T_(i/o) flowing over path P2.

SUMMARY

In light of the limitations associated with the prior art networktraffic distribution methods and in light of the limitations associatedwith the prior art ECMP routing techniques, it would be advantageous toimprove the distribution of network traffic in a manner that globally,with respect to a network system, mitigates traffic loss due to dynamicinstability in the system, and it would be advantageous to improve uponthe prior art methods for selecting the best path, among two or moreequal cost paths, over which to forward network traffic. According toone embodiment, a traffic distribution function running in each of aplurality of routers in a network system operates to apportion thedistribution of some or all of the traffic ingressing to the networksystem among two or more eligible paths in the network system byreceiving routing information necessary to calculate a set of two ormore eligible paths through the network system, receiving availablebandwidth information associated with each of the links connecting eachof the network devices to another network device in the system, andusing the received information to calculate the available bandwidthassociated with each one of the paths in the set of eligible paths,using the available path bandwidth information to calculate commonforwarding table entries which each of the plurality of routers use toupdate entries in their forwarding table, and each of the routerscomprising the network system apportioning the distribution of trafficingressing to them over the set of two or more eligible paths accordingto the bandwidth available on each path.

In another embodiment, traffic ingressing to each one of a plurality ofrouters comprising a network system is prioritized, and distributed bythe traffic distribution function over eligible paths in the networksystem according to its priority, with the highest priority trafficbeing distributed first and the traffic being distributed so that thereis minimal traffic loss.

In another embodiment, a packet network device comprising a networksystem receives a link state advertisement from one or more other packetnetwork devices in the network system, the link state advertisementincludes, among other things, a network interface index and bandwidth,interface type and path bandwidth, the packet network device accessesits forwarding table entries and determines that two or more equal costpaths can be selected over which to forward received network traffic,using the bandwidth information received in the link state advertisementto calculate a weighting for the two or more equal cost paths; andproportionately forwarding the received traffic over the two or moreweighted equal cost paths according to calculated path bandwidthweighting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a network system 100.

FIG. 2 is an illustration of an network system 200 that includes adistributed traffic distribution function.

FIG. 3 is a diagram of a router in the network system 200 withfunctional blocks that operate to support an embodiment of the trafficdistribution function.

FIG. 4 is a block diagram of a router showing an embodiment of a trafficdistribution function.

FIG. 5 is a block diagram of a router showing another embodiment of atraffic distribution function.

FIG. 6 is a block diagram of a router that includes a weighted ECMPfunction.

DETAILED DESCRIPTION

Cross Reference to Related Applications: This application claims thebenefit under 35 U.S.C. §119(e) of U.S. Provisional Patent ApplicationSer. No. 61/302,285 entitled “Weighted Equal Cost Multipath Method”,filed Feb. 8, 2010, the entire contents of which is incorporated hereinby reference.

FIG. 2 illustrates a network system 200 similar to the network system100 of FIG. 1. Network system 200 can be an autonomous system and it caninclude a plurality of edge routers, ER10-ERn, a plurality of corerouters, CR0-CRn, and the network system 200 in one embodiment caninclude a distributed Traffic Distribution Functionality (TDF) 201. In apreferred embodiment, each router (CR and ER) comprising the NS 200 caninclude the TDF 201. The network system 200 operates in a manner similarto that of network system 100 described earlier with reference to FIG.1, with the exception that the TDF 201 manages the global (networksystem wide) redistribution of traffic according to one or more trafficredistribution goals that the TDF 201 is configured to enforce. Eachinstance of the TDF 201 considers all of the traffic ingressing to theNS 200 when calculating one or more routes through the NS 200.Generally, the TDF 201 operates to continually collect/receive real-timeinformation associated with traffic (T_(i/o)) ingressing to andegressing from each ER in the system, to receive available bandwidthinformation associated with each link in the system, and to receive anindication of the volume of traffic flowing through each link in thesystem 200. In one embodiment, the TDF 201 that is included on each ofthe routers comprising system 200 can use the real-time information thatit receives from each of the other routers in the system 200 tocalculate the available bandwidth associated with all eligible pathsthrough the network system 200. Eligible paths in this case includepaths of equal or unequal cost, as calculated by an IGP running on eachof the network system 200 routers, over which traffic ingressing to theNS 200 can be forwarded to reach their proper destination (DA). The TDF201 can then use the available path bandwidth information to calculateFIB (forwarding information base) table entries that can be used toupdate existing forwarding table entries included on each of the routersin the NS 200. Each of the routers comprising the NS 200 can then usethe updated forwarding table entries to optimally redistribute some orall of the traffic flows ingressing to the network system 200 to any twoor more of the eligible paths through the system such that a minimaltraffic loss policy is enforced. According to an embodiment, based uponthe TDF 201 operation, NS 200 traffic flowing through some or all of theeligible paths in the NS 200 can be redistributed in a manner thatenforces a minimum traffic loss policy in the NS 200. For the purpose ofthis description, a traffic flow means traffic ingressing to the NS 200over any one or more of the routers comprising the system 200 and whichhave a common destination (DA).

According to another embodiment, the TDF 201 of FIG. 2 can be configuredto enforce a global policy for the redistribution of traffic thatminimizes traffic loss according to the priorities of individual trafficflows in the NS 200. A network administrator can select one or morepriority criteria that are used by the TDF 201 to determine how toassign network traffic to eligible paths in the NS 200, such thattraffic loss is minimized in the highest priority traffic first. The TDF201 can be configured to examine traffic flows for particularcharacteristics, which among other things can include suchcharacteristics as the bandwidth requirement of a flow, the ingressand/or egress router identity, the traffic pattern, and the amount oftraffic flowing through the routers. Depending upon the priority level(high to low) of a flow calculated by the TDF 201, the TDF can calculateforwarding table entries which biases the distribution of traffic,assigned different priority levels, to routes/paths that areundersubscribed or not.

A more detailed description of one embodiment will be undertaken withreference to FIG. 3, which is a diagram of a router 30 showingfunctionality that can be employed to support the TDF 201 described withreference to FIG. 2. For the purpose of this description, router 30represents any of the ERs and CRs comprising NS 200. The core/edgerouter 30 can include, among other functionality, a control module 31that is generally responsible for running management plane functionalityon the router, and one or more line cards (LC) 32 which are generallyresponsible for data plane functionality. Router 30 can also includeswitch fabric modules and other functional modules, but for the purposeof this description it is not important to describe their operation, andso they are not included in FIG. 3. The control module 31 can includeone or more route processor modules (RPMs) which generally operate torun network protocols necessary for the operation of the router 30 inthe network environment in which it is located. In this case, a singleRPM 33 is shown which can run a layer-3 interior gateway protocol (IGP)34, such as the well known Open Shortest Path First (OSPF) protocol orthe Intermediate System to Intermediate System (IS-IS) protocol. The IGP34 is comprised of a number of interdependent functions, such as a routeprocessing function, an extended link state advertisement (LSAx)function (described later), an ECMP function, and it includes a store ofstate information associated with each of the links in the NS 200. TheRPM also includes a forwarding information base (FIB) that is maintainedby a FIB manager operating in conjunction with the layer-3 networkprotocol, and the RPM includes a forwarding table manager sendsinformation and instruction to a forwarding table client function,running on the line card 32, which uses the information and instructionsto update appropriate entries in a forwarding table stored on the linecard 32.

Continuing to refer to FIG. 3, the router 30 also includes IP FlowInformation Export (IPFIX) protocol functionality and the TDF 210functionality alluded to earlier with reference to FIG. 2. The IPFIXprotocol is described in the publically available IETF RFC 5101specification. The IPFIX protocol generally operates to transmit IPtraffic flow information over the network, such as the NS 200 in FIG. 2.This IP traffic flow information can include the volume of trafficingressing to and egressing from one or more of routers in FIG. 2, itcan include the bandwidth availability on a particular NS 200 link, andit can include information associated with the volume of traffic beingtransmitted over a link. All of this bandwidth and traffic flowinformation can be included in IPFIX messages that are generated by theIPFIX protocol running on each of the routers comprising the NS 200.These IPFIX messages can be transmitted to all of the neighboringrouters in the NS 200. The format of these IPFIX messages is describedin RFC 5101. The RPM 33 can also include TDF 201 functionality whichwill be described later in detail with reference to FIG. 4, butgenerally employs a TDF processing function to operate on information ina store 41 to, among other things, calculate path bandwidth for each ofthe paths calculated by the IPG 34 of FIG. 3, to calculate linkutilization information that is maintained in the store 41 and to usethe results of these calculations to determine how to redistributetraffic in the NS 200.

Continuing to refer to FIG. 3, the IGP included on router 30 supportsthe transmission of link state advertisements (LSAs) to neighboringrouters in the NS 200. An LSA is employed by the OSPF protocol tocommunicate a routers local routing topology to all of the other routersdirectly connected to it. There are currently eleven different types ofLSAs, and one or more of these LSA types can be generated by the OSPFprotocol depending upon the needs of the network. According to anembodiment, the IGP in RPM 33 generates an LSA (can be type 9 opaque,type 10 opaque or type 11 opaque) that is extended (LSAx) to include,among other things, information associated with a path bandwidthcalculated by the router, an interface bandwidth (can be any one of aplurality of logical or physical interface bandwidths associated withthe router) and the identity or index of the interface, as well as theinterface type (physical, LAG, VLAN).

The line card 32 of FIG. 3 can be comprised of, among other things, oneor more I/O ports, packet processing functionality, memory in which tostore one or more forwarding tables and a forwarding table managerclient. The router 30 will typically include more than one line card,but for the purpose of this description, only one line card is shown inFIG. 3. The I/O ports operate, as a physical interface between therouter 30 and the network system 200, to transmit and to receiveinformation in various formats (typically in packet format) to and fromthe network system respectively. The ports send and receive thisinformation to and from the packet processor which generally operates toexamine the packets of information to determine how to forward them to anext hop in the network system. The information included in forwardingtable entries can be accessed by the packet process to make the next hopforwarding determination. An finally, the forwarding table managerclient receives instructions and information from the forwarding tablemanager in the RPM 33 that it uses to update entries in the forwardingtable in the line card 32.

The component parts comprising the Traffic Distribution Function (TDF)201 will now be described with reference to FIG. 4. As describedearlier, an instance of the TDF 201 runs on each of the routerscomprising the NS 200. The TDF 201 has access to a set of stores 41 thatinclude various global NS 200 bandwidth and traffic flow volumeinformation. All of the stores 41 included in each of the routers arecomprised of information that is substantially the same, and the stores41 can reside in memory associated with the CM 31 and are accessible byany of the functionality in the RPM 33. For the purpose of thisdescription, it is assumed that the TDF 201 has access to all of thestores 41, and the diagram in FIG. 4 shows each of the different storesof information 41 as being associated with the TDF 201. TDF 201 alsoincludes a TDF processing function 40 which is comprised of a trafficredistribution algorithm, a path bandwidth calculation function and alink utilization calculation function.

As described earlier with reference to FIG. 3, the RPM 33 maintainsvarious NS 200 information that is used to calculate path bandwidth anddetermine how traffic is distributed on a global basis with respect tothe NS 200. FIG. 4 includes a plurality of stores 41 where this NS 200information is maintained. One store includes bandwidth information foreach of the paths calculated by each of the routers in the NS 200 overwhich the routers can forward traffic. The paths calculated by each ofthe routers can be multiple, equal cost paths or not, the paths caninclude one or more links and the links can include one or more physicalor logical links. Another store includes bandwidth informationassociated with each of the physical or logical interfaces connected toa link. Each interface is designed to process a particular volume oftraffic, such a 1 Gbit/sec, and this store can include this type ofinformation. Another store includes the type of each interface(physical, VLAN, LAG) associated with the interface bandwidthinformation and the identify or index of the interface. Another storeincludes information associated with the volume of traffic ingress toand egressing from each of the ERs comprising the NS 200. A metric suchas bits, bytes or packets that are processed per second by the ER can bestored here. Another store includes the bandwidth that is available ateach of the links comprising the NS 200. Available link bandwidth forany particular link can be calculated by each router connected to thelink by subtracting the volume of traffic through a link at a point intime (or average vol. of traffic through a link over a period of time)from the total link bandwidth. Another store can include informationassociated with the volume of traffic passing over each of the links inthe system. And finally, another store can include informationassociated with bandwidth utilization of each of the links in the NS200. Specifically, this store includes two lists, a first list storesthe identifies of all links that are under-utilized (UULs), and a secondlist stores the identifies of all links that are over-utilized (OULs).

Continuing to refer to FIG. 4, the TDF processing function 40 running ineach instance of the TDF 201 on each router generally operates to useinformation in the stores to calculate available path bandwidth for eachof the paths that are calculated by the IGP 34 for the NS 200, it usesthe calculated available path bandwidth to calculate FIB entries that issends to the FIB manager, described with reference to FIG. 3. The TDFprocessing function 40 also includes a link utilization calculationfunction that operates to determine whether a link is under or overutilized and to maintain the link utilization store. Link utilization isdetermined by calculating whether the volume of traffic entering a link(link traffic volume or Tin) is greater than or less than the availablebandwidth to the link (Tin > or < Available Link Bandwidth). If Tin isgreater than the available link bandwidth, the link can be consideredover-utilized, and if Tin is less than the available link bandwidth, thelink can be considered under-utilized. More specifically, a networkadministrator can specify how much greater Tin is than the linkbandwidth before the link is over utilized, and vice versa. The pathbandwidth calculation function included in the TDF processing function40 uses Equation 1, below, to calculate the available bandwidth for eachof the paths known to the IGP running in each of the routers comprisingthe NS 200, and the path bandwidth is a function of the lowest availablelink bandwidth for each of the links in the path. So, for a path P1 thatincludes four links, L1-L4, the link with the lowest bandwidth is equalto the path bandwidth.

(For a path comprised of links 1, 2, 3 and 4) Path Bandwidth=Minimum(BWlink1, BWlink2, BWlink3, BWlink4) w/BWlinkn=available linkbandwidth  Equation 1

The individual path bandwidths calculated by the bandwidth calculationfunction using Equation 1 can be stored in the path bandwidth store andcan, separately or in combination with other path and link bandwidthinformation stored in or accessible to the TDF 201, be used by theredistribution algorithm in the TDF processing function 40 to calculateFIB table entries.

In operation, the TDF processing function 40 continually/periodicallyupdates the link utilization lists, it detects changes to link bandwidthavailability and calculates updated bandwidths for all of the pathsknown to the router in which the instance of the TDF 201 resides. Whenthe TDF 201 detects a change in a link bandwidth availability, itinvokes the redistribution algorithm in the TDF processing function 40to perform the following steps:

-   -   Generally: Compare the sum of the bandwidth (BWtotal) of a set        of multiple paths against the flow of Traffic (T_(i)) in to ER.        If BWtotal is greater the T_(i), then the operation of the TDF        201 can result in no traffic loss . . . otherwise Traffic loss        can be minimized.

For each router running TDF 201 in NS 200, check if any links ineligible paths comprising NS200 that are included in the listing of OUL.If so, then do the following:

-   1. ID OUL in each path, calculate how much traffic needs to be    redistributed . . . this calculation can be performed as follows:    -   Assuming that the Traffic T_(i) is being distributed equally        over each of the paths in the set of paths (ECMP), then for each        path, Redistributed Traffic (T_(r))=T_(i)/number of paths−path        bandwidth    -   So if T_(i) is 3 Gbps, and T_(i) is forwarded equally over each        of three paths, then the flow of traffic over each path is 1        Gbps. If for some reason, the available bandwidth for one of the        three paths decreases, due to the bandwidth available to a link        along the path decreasing, then TDF will detect that this link        is an OUL and perform the above calc.-   2. Deactivate/relax ECMP function.-   3. Adjust path bandwidth so that OUL becomes UUL, remove this OUL    from list.-   4. Identify paths with UULs, determine that path bandwidth is    underutilized and redistribute traffic calculated in #1 equally to    all of these paths without over utilizing any links . . . if this    causes a previously UUL to become OUL, then the redistribution of    T_(i) to this path is not permitted. In order to redistribute the    traffic, it is necessary to update the forwarding tables as follows.    Assuming that ECMPs are identified for a flow of traffic, that each    of the path costs have been calculated and that the total path    bandwidth is known, then for all ECMPs, OSPF can calculate how much    of the flow is distribute of each of the ECMPs as follows:    -   If there are n ECMPs d(P1-Pn) for a given network destination        address    -   And the respective path bandwidths are BW1-BWn for a total ECMP        bandwidth

(BW _(tot))=sum (BW1-BWn)

-   -   Find the % of the total bandwidth attributable to each path        using Equation 1:

% BW for a path Pn, BW'n=((BWn×100)/BW _(tot)),   Equation 1

BW'n is the percentage of the BW_(tot) that is apportioned to path Pn

-   -   If the maximum number of ECMPs the forwarding table can support        is Emax, then all of the BW, then use Equation 2 to determine        how to distribute the ECMPs across Emax.

ECMP % for path Pn, En=(BW'n×Emax)/100  Equation 2

-   -   OSPF than uses the information calculated in Equation 2 to        program the FIB.

-   5. If OULs traffic is successfully redistributed (no UULs become    OULs), then remove it from OUL list [If the TDF determines that the    link is over utilized by 250 Mbps, then TDF will attempt to    redistribute this amount of traffic in T_(i) equally over each of    the other paths in the set of paths].

-   6. Remove any UULs from list that are no longer underutilized after    the redistribution

-   7. If there are no UULs left in any paths, then process terminates    in this router and another router can run the process

-   8. Update the path bandwidth store to reflect any changes to the    path bandwidths as the result of the redistribution.

Operation of the traffic redistribution function 201 can result in theredistribution of one or more traffic flows in NS 200. For instance, ifas a result of the redistribution of a first flow of traffic over afirst path, a link comprising a second path may become underutilized(UUL). In this event, TDF 201 try to redistribute traffic to this UUL.TDF 201 continually monitors information received from the NS 200 andattempts to redistribute traffic entering NS 200 in an optimal manner inorder to enforce the minimum traffic loss policy.

FIG. 5 is a diagram showing functionality and information stores thatcan be employed in another embodiment of a TDF 501. In a preferredembodiment, the TDF 501 functionality is distributed and can be includedin each of the routers (ERs and CRs) comprising the NS 200 of FIG. 2.However, in contrast to the TDF 201 described earlier, TDF 501 isconfigured to redistribute NS 200 traffic so as to minimize traffic lossaccording to the priority of the traffic ingressing to a router. Withthe exception of the traffic priority store included in the store 51 andthe traffic priority calculation function included in the TDF processingfunction 50, the TDF 501 operates in much the same manner as the TDF 201described earlier with reference to FIG. 4. The redistribution algorithmin this embodiment is designed to attempt to first redistribute thehighest priority traffic over paths in which each of the linkscomprising the path are undersubscribed, and then attempt toredistribute lower priority traffic. In order to enforce the policy(minimization of traffic loss according to traffic priority) for whichthe TDF 501 is configured, it may be necessary for the TDF 501 toredistribute some lower priority traffic through oversubscribed pathswhich can result in some traffic loss for these flows. The TDF 501 isconfigured with one or more traffic prioritization criteria, which caninclude, but not limited to, the bandwidth needed to support the flow oftraffic (traffic bandwidth), traffic cost (number of hops from ingressrouter to destination), and the identity of the router into which thetraffic ingresses or from which it egresses. These trafficprioritization criteria can be stored in memory associated with ofaccessible by the CM 31 described earlier with reference to FIG. 3. Forthe purpose of this description, these prioritization criteria areincluded in the stores 51.

In operation, the TDF processing function 50, running on each router inNS 200, continually/periodically updates the link utilization lists, itdetects changes to link bandwidth availability and calculates updatedbandwidths for all of the eligible paths in the NS 200. The TDFprocessing function 50 running on each router also continually receivesinformation relating to traffic ingressing to each of the routers in theNS 200, and the traffic priority calculation function in the TDFprocessing function 50 uses the traffic priority criteria in the store51 to calculate a traffic priority for the flow and to store thistraffic priority in the traffic priority store. When the TDF 501 detectsa change in a link bandwidth availability, it invokes the redistributionalgorithm in the TDF processing function 50 to perform the followingsteps:

-   -   Generally: Compare the sum of the bandwidth (BWtotal) of a set        of multiple paths against the flow of Traffic (Ti) in to ER. If        BWtotal is greater the Ti, then the operation of the TDF 201 can        result in no traffic loss . . . otherwise Traffic loss can be        minimized.    -   For each router running TDF in NS 200 (and starting with the        highest priority traffic), check if any links in eligible paths        comprising NS 200 that are included in the listing of OUL. If        so, then do the following:

-   1. ID OUL in each path, calculate how much traffic needs to be    redistributed . . . this calculation can be performed as follows:    -   Assuming that the Traffic T_(i) is being distributed equally        over each of the paths in the set of paths (ECMP), then for each        path, Redistributed Traffic (T_(r))=T_(i)/number of paths−path        bandwidth

So if T_(i) is 3 Gbps, and T_(i) is forwarded equally over each of threepaths, then the flow of traffic over each path is 1 Gbps. If for somereason, the available bandwidth for one of the three paths decreases,due to the bandwidth available to a link along the path decreasing, thenTDF will detect that this link is an OUL and perform the above calc.

-   2. Deactivate/relax ECMP function.-   3. Adjust path bandwidth so that OUL becomes UUL, remove this OUL    from list.-   4. Identify paths with UULs, determine that path bandwidth is    underutilized and redistribute traffic calculated in #1 equally to    all of these paths without over utilizing any links . . . if this    causes a previously UUL to become OUL, then the redistribution of    T_(i) to this path is not permitted. In order to redistribute the    traffic, it is necessary to update the forwarding tables as follows.    Assuming that ECMPs are identified for a flow of traffic, that each    of the path costs have been calculated and that the total path    bandwidth is known, then for all ECMPs, OSPF can calculate how much    of the flow is distribute of each of the ECMPs as follows:    -   If there are n ECMPs d(P1-Pn) for a given network destination        address    -   And the respective path bandwidths are BW1-BWn for a total ECMP        bandwidth (BW_(tot))=sum (BW1-BWn)    -   Find the % of the total bandwidth attributable to each path        using Equation 1:

% BW for a path Pn, BW'n=((BWn×100)/BW _(tot)),   Equation 1

BW'n is the percentage of the BW_(tot) that is apportioned to path Pn

-   -   If the maximum number of ECMPs the forwarding table can support        is Emax, then all of the BW, then use Equation 2 to determine        how to distribute the ECMPs across Emax.

ECMP % for path Pn, En=(BW'n×Emax)/100  Equation 2

-   -   OSPF than uses the information calculated in Equation 2 to        program the FIB.

-   5. If OULs traffic is successfully redistributed (no UULs become    OULs), then remove it from OUL list [If the TDF determines that the    link is over utilized by 250 Mbps, then TDF will attempt to    redistribute this amount of traffic in T_(i) equally over each of    the other paths in the set of paths].

-   6. Remove any UULs from list that are no longer underutilized after    the redistribution

-   7. If there are no UULs left in any paths, then process terminates    with respect to the flow of traffic and the TDF 501 attempts to    redistribute a flow of lower priority. Otherwise another router can    run the process.

-   8. Update the path bandwidth store to reflect any changes to the    path bandwidths as the result of the redistribution.

As with the TDF 201, traffic redistribution according to the trafficredistribution policy enforced by each instance of TDF501 running in onerouter in NS 200 can result in the redistribution of traffic over one ormore other eligible paths in the NS 200. The TDF 201 continuallymonitors information received from the NS 200 and attempts toredistribute traffic entering the NS 200 in a manner that enforces thetraffic prioritized minimum loss policy.

Referring again to FIG. 1, and as described earlier in the Background,some or all of the routers, ER1-ERn and CR0-CRn, can run a networklayer-3 routing protocol, such as the well known OSPF protocol. OSPFuses a cost metric associated with each router interface (physical orlogical) to calculate one or more shortest paths from the router to adestination. The cost metric can be assigned to each interface by asystem administrator (or automatically) and this cost metric can bedependent on the distance from one router to another (round-trip time),link bandwidth, link availability (delay), and/or link reliabilityfactors to name only four criteria that can be considered when assigningcost to a router interface. The OSPF protocol running on a router usesthe costs assigned to each of its interfaces to calculate the shortestpaths from it to a destination address, for instance. Specifically, thewell known Dijkstra algorithm can be used to calculate the least costpaths through a network system, such as the network system 100 inFIG. 1. The result of applying the Dijkstra algorithm to link stateinformation maintained by each router is a series of connected routersthat represent the least cost paths to each router and the cost of eachpath.

Continuing to refer to FIG. 1, if the result of the calculation toidentify the least cost paths from ER1 to ER3 in the NS 100 results in apath P2 cost equal to 3.0 and a path P3 cost equal to 3.0, then OSPFrunning on ER1 can use the well known Equal Cost Multi-Path (ECMP)routing technique to distribute the traffic Ti evenly/symmetricallybetween path P2 and path P3 (assuming P2 and P3 have enough availablebandwidth to support the traffic). ECMP is a routing technique that isexplicitly supported by the OSPF protocol. A number of different methodscan be used to determine which of several equal cost paths or next hopsare selected. Hash-threshold is one method for determining which ofseveral equal cost next hops to select and the round-robin method isanother. Each method has their advantages and disadvantages and thereasons for selecting one of the other method is not discussed here.ECMP routing techniques typically divide the traffic with a commondestination equally among the multiple equal cost paths, regardless ofthe bandwidth that is available on any one of the equal cost paths andregardless of the technique employed to select the traffic transmissionpath.

Continuing to refer to FIG. 1, if it is assumed, as described above,that the traffic Ti is being forwarded over the two equal cost paths,paths P2 and P3, that the Ti volume is 2 Gbps, that the availablebandwidth on path P2 is 1 Gbit/second and that the available bandwidthon path P3 is 2 Gbits/second. The ECMP routing technique operates toevenly forward Ti over each of the two paths, which results in 1 Gbps ofTi traffic flowing through path P1 and 1 Gbps of Ti traffic flowingthrough path P2. In this case, path P1 is nearly oversubscribed and pathP2 is undersubscribed Again, assuming that link L5 is a logicalcombination of multiple physical links, and in the event that one of thephysical links comprising link L5 flaps, L5 can become oversubscribedand some of the Ti traffic can be dropped.

It was discovered that the ECMP routing technique can be modified toconsider available link bandwidth and path bandwidth (BWp) independentlyof path cost when distributing traffic to equal cost paths. Thistechnique is referred to as Weighted Equal Cost Multi-Path (WECMP)routing, and it can be employed by one or more of the routers in the NS100 of FIG. 1 to distribute traffic ingressing to the routersproportionately according to the bandwidth of each of the paths overwhich the traffic is distributed. As the result of employing the WECMProuting technique, it is possible to decrease the number ofover-subscribed paths in the network, which has the effect of minimizingtraffic lost due to over subscription.

FIG. 6 is a diagram illustrating functional elements that can comprise arouter 60 according to an embodiment. For the purpose of thisdescription, router 60 represents any one or more of the CRs or ERscomprising the NS 200 of FIG. 2. Router 60 is comprised of a controlmodule 61 that is generally responsible for running management planefunctionality on the router, and one or more line cards (LC) 66 whichare generally responsible for data plane functionality. Router 60 canalso include switch fabric modules and other functional modules, but forthe purpose of this description it is not important to describe theiroperation and so they are not included in FIG. 6. The control module 61can include one or more route processor modules (RPMs) which generallyoperate to run network protocols necessary for the operation of therouter 60 in the network environment in which it is located. In thiscase, a single RPM 62 is shown which can run a layer-3 interior gatewayprotocol (IGP) 63, such as the well known Open Shortest Path First(OSPF) protocol or the Intermediate System to Intermediate System(IS-IS) protocol. The IGP 63 is comprised of a number of interdependentfunctions, such as a route processing function, an extended link stateadvertisement (LSAx) function 64, a WECMP function 65, and it includes astore of state information associated with each of the links in the NS200. The RPM 62 also includes a forwarding information base (FIB) thatis maintained by a FIB manager operating in conjunction with the layer-3network protocol, and the RPM includes a forwarding table manager sendsinformation and instruction to a forwarding table client function,running on the line card 66, which uses the information and instructionsto update appropriate entries in a forwarding table stored on the linecard.

Continuing to refer to FIG. 6, and as described above, the IGP 63included on router 60 supports the transmission of extended link stateadvertisements (LSAx) to neighboring routers in the NS 200. An LSA isemployed by the OSPF protocol to communicate a routers local routingtopology to all of the other routers directly connected to it. There arecurrently eleven different types of LSAs, and one or more of these LSAtypes can be generated by the OSPF protocol depending upon the needs ofthe network. According to an embodiment, the IGP in RPM 62 generates anLSA (can be type 9 opaque, type 10 opaque or type 11 opaque) that isextended (LSAx) to include, among other things, information associatedwith a path bandwidth calculated by the router (for example the LSAxfunction can include a path bandwidth calculation routine), an interfacebandwidth (can be any one of a plurality of logical or physicalinterface bandwidths associated with the router), the identity or indexof the interface, and the interface type (physical, LAG, VLAN). The pathbandwidth information calculated by the LSAx function 64 (in this case)can be stored in the link state store on the RPM 62.

Generally, each line card 66 in router 60 is configured to support amaximum number of ECMPs. This support is typically provided byprogramming the line card 66 forwarding table such that an equal numberof table entries are programmed to forwarding a traffic flow on two ormore ECMPs. For instance, if first and second equal cost paths areassigned to receive traffic from a particular flow, and if theforwarding table is configured to support six ECMPs, then three entriesin the table could be programmed to forward half of the traffic over thefirst path, and three entries can be programmed to forward half of thetraffic over the second path. According to an embodiment, WECMP 65 usespath bandwidth information associated with each one of two or more equalcost paths in a set of equal cost paths (the set of equal cost paths arededicated to a single traffic flow) to calculate how much traffic can beforwarded over each path in proportion to the paths bandwidth.

The WECMP 65 functionality included in OSPF 63 of RPM 62 includes a pathdistribution algorithm 67 that operates, using the path bandwidthsassociated with each equal cost path in a set of equal cost paths, todetermine the proportions (by volume) of a traffic flow that will beforwarded over each one of the paths in the set of paths. The output ofthe path distribution algorithm 67 can be used by the IGP function 63 toupdate the FIB. The path distribution algorithm operates as follows.

Assuming that a router has identified ECMPs for a flow of traffic, thateach of the path costs have been calculated and that the total pathbandwidth is known, then for all ECMPs, OSPF can employ WECMP and theindividual path bandwidths to calculate how much of the flow isdistribute of each of the ECMPs as follows:

-   -   If there are n ECMPs d(P1-Pn) for a given network destination        address    -   And the respective path bandwidths are BW1-BWn for a total ECMP        bandwidth (BWtot)=sum (BW1-BWn)    -   Find the % of the total bandwidth attributable to each path        using Equation 1:

% BW for a path Pn, BW'n=((BWn×100)/BWtot),   Equation 1

BW'n is the percentage of the BWtot that is apportioned to path Pn

-   -   If the maximum number of ECMPs the forwarding table can support        is Emax, then all of the BW, then use Equation 2 to determine        how to distribute the ECMPs across Emax.

ECMP % for path Pn, En=(BW'n×Emax)/100  Equation 2

The following is an example of the operation of the path distributionalgorithm 67. Given 2 ECMPs for a destination address, path P1 and pathP2, and P1 BW is 1 Gbps and P2 BW is 2 Gbps for a total ECMP BW ofBWtot=3 Gbps. Then, according to Eq. 1: BW'1=33.3% and BW'2=66.6%. IfEmax is 6, then using Eq. 2, the number of ECMPs used to distribute pathP1 bandwidth is 2 and the number of ECMPs used to distribute path P2bandwidth is 4.

The forgoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforgoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

1. A method of forwarding traffic in a network system, comprising: apacket network device receiving a link state advertisement from one ormore other packet network devices in the network system, the link stateadvertisement comprising a network interface index and bandwidth,interface type and path bandwidth; the packet network device accessingits forwarding table entries and determining that two or more equal costpaths can be selected over which to forward received network traffic;using the bandwidth information received in the link state advertisementto calculate a weighting for the two or more equal cost paths; andproportionately forwarding the received traffic over the two or moreweighted equal cost paths according to calculated path bandwidthweighting.